For me, I think, it's a variant form of the earworm - the tune sticks in my head, and words just kind of attach themselves to it as an alternative to rumty-tumty-tum.

Another example is Gamble Gold as recorded by Steeleye Span (Womble Period), which I find extremely memorable - in fact, so memorable it's downright irritating. At least, it's extremely memorable apart from the words, which go something like
Ye fiddlers three
How well ye be
With a pint or three
When it's time for tea...